Filtering in the RF front end of a base station, e.g., a Long Term Evolution (LTE) eNodeB, has demanding requirements that include high-order filtering, high dynamic range, and low loss. Conventional RF front end filters for base station transceivers are high order band pass filters constructed with multiple high quality resonators. These filters are not electronically reconfigurable and are relatively bulky. With the adoption of multiple antennas and transceiver architectures, it is desirable that these RF front end filters be reduced in size. Further, vendors of base stations must offer front end filtering systems that operate in frequencies that their customers use. This results in a large ensemble of potential filter specifications. If the filters of an RF front end were reconfigurable, then a single RF front end could meet the frequency specifications of multiple customers and/or multiple standards. Further, a cognitive radio—a radio that adapts to its environment—requires frequency agility.
Continuous-time (analog) filters employ RLC (resistor-inductor-capacitor) resonators with tunable reactive components. Many of these filters exhibit low Q. Other filters use high Q components but their integration degrades the overall Q of the filter. Some filters use switches to connect or disconnect high Q components, but these filters have limited reconfigurability and tend to have high insertion loss.
Discrete time filters typically have multiple paths, where each path has a different time delay and complex gain. Some digital filters use tunable phase shifters, attenuators, or vector modulators in each path. High-order analog discrete-time filtering requires a large number of paths, each path having frequency dependent behavior which drifts with environment and age. Further, high-order filtering requires many wideband splitters and combiners. The monitoring circuitry for compensating the drift, the RF delays for each path, and the splitters and combiners occupy much physical space.
An exemplary known frequency agile RF front end is shown in FIG. 1. FIG. 1 shows an RF feed forward (FF) transmitter 10 that includes a power amplifier 12 that provides an information signal to a first coupler 14. The transmitter of FIG. 1 has a digital signal processing (DSP) path in parallel with an RF path. The RF path includes a delay line 16 to provide a delay that is comparable to a delay associated with the DSP path. The DSP path includes an amplifier 18, a notch filter 20, a frequency down converter 22, a low pass filter 24, an analog-to-digital converter (ADC) 26, a digital filter 28, a digital-to-analog converter (DAC) 30, a low pass filter 32, a frequency up converter 34, and an amplifier 36. A function of the DSP FF path is to suppress the in-band components of the signal so that when the signal in the DSP FF path is added to the signal in the RF path, out-of-band components of the signal will cancel and in-band components of the signal will be passed to an antenna 42.
The output of the amplifier 36 is input to a coupler 38 which also receives the output of the delay line 16. Ideally, signals outside an RF pass band are suppressed by cancellation in the coupler 38. The output of the coupler 38 is input to a next coupler 40 which couples the RF signal to an antenna 42. The coupler 40 also couples the RF signal to a feedback path. The feedback path includes an amplifier 44, a frequency down converter 46, a filter 48, an ADC 50, and a digital signal processor (DSP) 52. A function of the feedback path is to provide adaptation of the coefficients of the digital filter 28 based on the output of the coupler 38 which combines the signals of the RF FF path and the DSP FF path.
In a feed forward (FF) architecture such as that shown in FIG. 1, the attenuation bandwidth is limited by the group delay mismatch between the two FF paths. The group delay of the RF FF path is substantially equal to the delay of the delay line 16. The group delay of the DSP FF path is the sum of the group delays of the RF and analog components in the path, as well as the pipeline latencies within the digital components, including the ADC 26, digital filter 28 and DAC 30.
A typical group delay of the DSP FF path of FIG. 1 may be on the order of 50-100 nanoseconds over the operating bandwidth of the front end 10. One solution to correct the mismatch between the two FF paths is to increase the delay of the delay line 16. For example, inserting about 7.8 meters of coaxial cable can be expected to reduce the delay mismatch by about 37 nanoseconds. Problems with this solution are that the long delay line adds costs, consumes space and has an insertion loss that increases with length.
What is desired is a reconfigurable compact filter that can perform high order filtering while meeting typical bandwidth and insertion loss constraints in a base station front end.